Feature request: reference original root element in sub_expression or pipe

[MountLion]

I want get "beta" from following json but cannot:
{ "data":{ "a": "alpha", "b": "beta" }, "key": "b" }
What if key | $root.data.@ ?

[springcomp]

@MountLion we used to think about specifying the $ token for this use case. However, it seems better served by the let() function instead.